KIR3DS1 is presumed to be an NK cell activation receptor. The gene has many (&gt;40) inhibitory alleles, known as KIR3DL1. Various KIR3DL1 subtypes have been shown to interact directly with HLA-I proteins with the Bw4 public epitope. Regardless of its similarity to KIR3DL1, and its established role in HIV disease, KIR3DS1 has only recently been shown to be expressed on NK cells and no ligand has been described. Toward an understanding of KIR3DS1 ligand binding we developed a reporter system for the engagement of KIR3DS1. We continue to dissect the biochemical basis of the apparent lack of KIR3DS1 binding to HLA-Bw4. KIR3DL1 binds to HLA-Bw4 but the highly related KIR3DS1 does not. Despite the lack of direct evidence for interaction, genetic association studies suggest that carriage of KIR3DS1 and Bw4 provides strong protection from the advancement of HIV disease. We have carried out targeted mutagenesis that defines a critical role for Glycine 138 of KIR3DL1 for maintaining HLA-Bw4 binding. In addition, we are collaborating with a group in the process of solving the crystal structure of KIR3DL1 and testing the role that individual residues play in the interactions between this receptor and HLA-Bw4. Lastly, we continue to dissect the biochemical basis of the apparent lack of KIR3DS1 binding to HLA-Bw4. KIR3DL1 binds to HLA-Bw4 but the highly related KIR3DS1 does not. Despite the lack of direct evidence for interaction, genetic association studies suggest that carriage of KIR3DS1 and Bw4 provides strong protection from the advancement of HIV disease. We have carried out targeted mutagenesis that defines a critical role for Glycine 138 of KIR3DL1 for maintaining HLA-Bw4 binding. In addition, we are collaborating with a group in the process of solving the crystal structure of KIR3DL1 and testing the role that individual residues play in the interactions between this receptor and HLA-Bw4. Mutation of the residues implicated by the crystal structure has provided significant insight into the mechanisms of recognition and peptide specificity of KIR3DL1. By comparison, these data have provided important clues for our understanding of KIR3DS1. We have now begun to extend this analysis to KIR3DL2, a related receptor implicated in ankylosing spondylitis and cutaneous lymphoma.